Microfluidic devices require that liquid flow easily pass through the channels and that non-specific adsorption of reagents and analytes should be as low as possible, i.e. insignificant for the reactions to be carried out.
Reagents and/or analytes includes proteins, nucleic acids, carbohydrates, cells, cell particles, bacteria, viruses etc. Proteins include any compound exhibiting poly- or oligopeptide structure.
The hydrophilicity of surfaces within microchannel structures shall support reproducible and predetermined penetration of an aqueous liquid into the various parts of a structure. It is desirable that once the liquid has passed a possible break at the entrance of a part of the structure then the liquid spontaneously shall enter the part by capillary action (passive movement). This in turn means that the hydrophilicity of the surfaces within microchannel structures becomes of increasing importance when going from a macroformat to a microformat.
From our experience, water contact angles around 20 degrees or lower may often be needed to accomplish reliable passive fluid movement into microchannel structures. However, it is not simple to manufacture surfaces which permanently have such low water contact angles. There is often a tendency for a change in water contact angles during storage, which renders it difficult to market microfluidic devices having standardised flow properties.
The situation is complicated by the fact that methods for preparing surfaces with very low water contact angles do not necessarily reduce the ability to non-specifically adsorb reagents and sample constituents. The surface/volume ratio increases when going from a macroformat down to smaller formats. This means that the capacity for non-specific adsorption of a surface increases inversely with the volume surrounded by the surface. Non-specific adsorption therefore becomes more critical in microformat devices than in larger devices.
An unacceptable non-specific adsorption of biomolecules is often associated with the presence of hydrophobic surface structures. This particular problem therefore is often more severe in relation to surfaces made of plastics and other hydrophobic materials compared to surfaces of native silicon surfaces and other similar inorganic materials.
There are a number of methods available for treating surfaces to make them hydrophilic in order to reduce non-specific adsorption of various kinds of biomolecules and other reagents. However, these methods generally do not concern balancing a low non-specific adsorption with a reliable and reproducible liquid flow when miniaturizing macroformats down into microformats. Compare for instance Elbert et al., (Annu. Rev. Mater. Sci. 26 (1996) 365-394).
Surfaces that have been rendered repelling for biopolymers in general by coating with adducts between polyethylenimines and hydrophilic polymers have been described during the last decade (Brink et al (U.S. Pat. No. 5,240,994), Bergström et al., U.S. Pat. No. 5,250,613; Holmberg et al., J. Adhesion Sci. Technol. 7(6) (1993) 503-517; Bergström et al., Polymer Biomaterials, Eds Cooper, Bamfors, Tsuruta, VSP (1995) 195-204; Holmberg et al., Mittal Festschrift, Eds Van Ooij, Anderson, VSP 1998, p 443-460; and Holmberg et al., Biopolymers at Interfaces, Dekker 1998 (Surfactant Science Series 75), 597-626). Sequential attachment of a polyethylenimine and a hydrophilic polymer has also been described (Kiss et al., Prog. Colloid Polym. Sci. 74 (1987) 113-119).
Non-specific adsorption and/or electroendosmosis have been controlled in capillary electrophoresis by coating the inner surface of the capillary used with a hydrophilic layer, typically in form of a hydrophilic polymer (e.g. van Alstine et al U.S. Pat. No. 4,690,749; Ekström & Arvidsson WO 9800709; Hjertén, U.S. Pat. No. 4,680,201 (poly methacrylamide); Karger et al., U.S. Pat. No. 5,840,388 (polyvinyl alcohol (PVA)); and Soane et al., U.S. Pat. No. 5,858,188 and U.S. Pat. No. 6,054,034 (acrylic microchannels). Capillary electrophoresis is a common name for separation techniques carried out in a narrow capillary utilizing an applied electric filed for mass transport and separation of the analytes.
Larsson et al (WO 9958245, Amersham Pharmacia Biotech) presents among others a microfluidic device in which microchannels between two planar substrates are defined by the interface between hydrophilic and hydrophobic areas in at least one of the substrates. For aqueous liquids the hydrophilic areas define the fluid pathways. Various ways of obtaining a pattern of hydrophobic and hydrophilic surfaces for different purposes are discussed, for instance, plasma treatment, coating a hydrophobic surfaces with a hydrophilic polymer etc. The hydrophilic coat polymers suggested may or may not have aryl groups suggesting that Larsson et al are not focusing on lowering the water contact angle as much as possible or avoiding non-specific adsorption.
Larsson, Ocklind and Derand (PCT/EP00/05193 claiming priority from SE 9901100-9, filed Mar. 24, 1999) describe the production of highly hydrophilic surfaces made of plastics. The surfaces retain their hydrophilicity even after being in contact with aqueous liquids. An additional issue in PCT/EP00/05193 is to balance a permanent hydrophilicity with good cell attachment properties. The surfaces are primarily suggested to be used in microfabricated devices.
Polyethylene glycol has been linked directly to the surface of a microchannel fabricated in silicone for testing the ability of polyethylyne glycol to prevent protein adsorption. See Bell, Brody and Yager (SPIE-Int. Soc. Opt. Eng. (1998) 3258 (Micro- and Nanofabricated Structures and Devices for Biomedical Environmental Applications) 134-140).